Reverse osmosis is the closest thing a household can get to pure water without boiling it. Line pressure forces water against a semi-permeable membrane that passes water molecules and rejects the great majority of dissolved salts, metals and larger organic molecules, which leave with the reject stream. The result is water with very little dissolved in it, which is why it tastes different and why it behaves differently in appliances.
How pure is pure, in practice
A working membrane rejects the large majority of dissolved solids in a single pass, and the post-carbon polishing filter that follows it cleans up taste and any small molecules that carried through. What it does not deliver is laboratory-grade purity: that requires multiple passes, deionization resin or distillation, none of which belong in a kitchen cabinet. For household purposes the practical measure is total dissolved solids at the faucet compared with the feed, which a cheap handheld meter reads in seconds. Track that ratio over time and you have a direct read on membrane health rather than an opinion about it.
The stages around the membrane
The membrane never works alone. A sediment prefilter takes out grit that would otherwise abrade and blind the surface. One or two carbon prefilters strip chlorine, which chemically attacks thin-film composite membranes and is the single most common cause of premature failure on city water. After the membrane comes storage, then a final carbon polish on the way to the faucet to remove any taste the tank imparts. Each of those stages is a consumable on its own interval, and the membrane outlasts all of them provided the prefilters are actually changed rather than merely intended to be.
What reverse osmosis does not do
It does not disinfect. A membrane is a physical barrier and a well-maintained system rejects organisms, but a compromised o-ring or a hairline defect makes that barrier unreliable, which is why a well with a bacteria history gets ultraviolet treatment or chlorination rather than relying on a membrane. It does not remove dissolved gases well, so hydrogen sulfide odor passes through. And it treats only the tap it is plumbed to, so hardness, iron and scale everywhere else in the house are untouched. Our reverse osmosis comparison covers matching a system to what the water report actually shows.
Signs the membrane is on the way out
Watch for three things. Total dissolved solids at the faucet creeping toward the feed reading means rejection is falling. A tank that takes noticeably longer to fill than it used to, with feed pressure unchanged, means the membrane is fouling. And taste returning after being clean for years is the same story arriving through your senses rather than a meter. None of these is sudden, which is why a periodic reading logged somewhere beats waiting to notice. Before condemning a membrane, replace the prefilters and check the tank air charge, since both mimic membrane failure.
Questions people ask about pure water reverse osmosis
Is reverse osmosis water the same as distilled water?
Both are very low in dissolved solids but they are made differently. Distillation boils and condenses in batches; reverse osmosis pushes water through a membrane continuously and sends a reject stream to drain.
Does reverse osmosis remove bacteria?
A sound membrane rejects them, but it is not certified disinfection and a small defect makes it unreliable. Wells with a bacteria history need ultraviolet or chlorination as the actual barrier.
Why does my reverse osmosis faucet run slowly?
Usually a storage tank that has lost its air charge, or clogged prefilters starving the membrane. Check the tank pressure with the tank empty before assuming the membrane has failed.
Is reverse osmosis water bad for pipes?
It is slightly aggressive toward some metals, which is why it is delivered through its own plastic tubing and dedicated faucet rather than plumbed back into the household copper.